This invention relates to a rotation mechanism and, in particular, to a rotation mechanism for use in rotating an element which is located within a pressure containing housing.
The specific application of the present invention is to a type of wellhead system known as “Tie back” which is used on wells with pre-drilled casing strings which are suspended at the seabed or mud line. Whilst specifically applicable to this type of wellhead system, the invention may be used in other applications in which rotation of an element in a pressure containing housing is required.
In order to connect pre-drilled casing strings to a wellhead at the surface, which is typically located on a production platform, a means has to be provided in the surface wellhead to place the mudline to surface (tie back) casing strings in tension.
This requires that the casing suspension mechanism (hanger) within the wellhead provides a means of adjustment which conventionally comprises a threaded hanger landing ring. This ring is mounted on the casing hanger and is rotated, such that it is moved axially down the body of the casing hanger whilst the hanger is held at the required tension by tubular members that pass back to the surface. The landing ring is rotated by means of an engaging tool, which is in turn rotated by tubular members passing back to the surface, these tubular members being external to and concentric with the internal tubular member which are holding the casing hanger in tension. The landing ring is rotated down the body of the casing hanger until it abuts a landing shoulder such that the applied tension on the hanger cannot be released.
However, such an arrangement introduces problems in handling, given the concentric tubular members, and increases the number of manipulations down the well (i.e. trips with a tool) which adds significant cost to the drilling operation.
Accordingly, the present invention aims to provide a system which eliminates the requirement for an external tubular member to provide the means of rotating the threaded hanger landing ring, but which still maintains the pressure integrity of the housing.